mbuf: fix C++ compatibility by making sched struct visible

mbuf: fix C++ compatibility by making sched struct visible

Although C compilation works with the struct rte_mbuf_sched
declared inside the struct rte_mbuf namespace, C++ fails to
compile. This fix moves the rte_mbuf_sched struct up to the
global namespace, instead of declaring it inside the struct
mbuf namespace.

The struct rte_mbuf_sched is being used on the stack in
rte_mbuf_sched_get() and as a cast in _set(). For this
reason, it must be exposed as an available type.

Fixes: 5d3f72100904 ("mbuf: implement generic format for sched field")

Signed-off-by: Harry van Haaren <harry.van.haaren@intel.com>
Acked-by: Cristian Dumitrescu <cristian.dumitrescu@intel.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>